1WGA J2223.7-0206: A Narrow-Line Quasi-Stellar Object in the XMM-Newton field of view of 3C 445

Abstract

We report the discovery of a Narrow Line QSO located at about 1.3' from the Broad Line Radio Galaxy 3C 445. The source, 1WGA J2223.7-0206, although already revealed by ROSAT, has never been optically identified previously. An XMM-Newton observation of 3C 445 has allowed, for the first time, an accurate X-ray spectral study of 1WGA J2223.7-0206, revealing an ultra-soft spectrum and fast flux variations typical of Narrow Line AGN. The 0.2-10 keV spectrum is well represented by a power law (Γ=2.5) plus a black body component (kT = 117 eV) absorbed by Galactic N_H. About 80% of the X-ray flux (F_{0.2-10 keV} ∼ 3 × 10^-13 erg cm^-2 s^-1) is emitted below 2 keV. The 0.2-2 keV flux is observed to decrease by about a factor 1.6 in about 5000 s.

The optical observations, triggered by the X-ray study, confirm the Narrow Line AGN nature of this source. The continuum is blue with typical AGN emission lines, pointing to a redshift z=0.46. The full width half maximum of H_β and the flux ratio [O III]/H_β are 2000 km s^-1 and 0.21, respectively. The optical luminosity (M_R=-23.2) and the point-like appearance in the optical images identify 1WGA J2223.7-0206 as a Narrow Line QSO.

From the optical-UV-X-ray Spectral Energy Distribution we obtain a lower limit of the bolometric luminosity of 1WGA J2223.7-0206 (L_bol ≥3 × 10⁴⁵ erg s^-1) implying, for accretion rates close to the Eddington limit, a black hole mass M_BH ≥2 × 10⁷ M_⊙.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA), with the Bologna Astronomical Observatory in Loiano, Italy, and with the WHT telescope of the ING at La Palma, Spain.